1. Field of the Invention
The present invention relates to an optical article comprising on its rear face, and optionally on its front face, an antireflective coating which strongly reduce reflection in the UVA- and UVB-radiation range, and in the visible region. The optical article may especially be an ophthalmic lens, especially a tinted solar lens.
2. Description of Related Art
The solar spectrum comprises electromagnetic radiations having various wavelengths, especially ultraviolet radiation (UV). The UV spectrum has many bands, especially UVA, UVB and UVC bands. Amongst those UV bands which do reach the earth surface, UVA band, ranging from 315 nm to 380, and UVB band, ranging from 280 nm to 315 nm, are particularly harmful to the retina.
Traditional antireflective coatings are designed and optimized to reduce reflection on the lens surface in the visible region, typically within the spectrum range of from 380 to 780 nm. As a rule, the reflection in the ultraviolet region (280-380 nm) is not optimized, and is frequently reinforced by the traditional antireflective coating itself. The article “Anti-reflective coatings reflect ultraviolet radiation”, Citek, K. Optometry 2008, 79, 143-148 underlines this phenomenon.
The mean reflection in the UVA and UVB regions may thus attain high levels (up to 60%) for traditional antireflective lenses. For example, as regards non-solar antireflective articles which are marketed by most of the manufacturers over the course of these recent years, the UV mean reflection does range from 10 to 25%, for an angle of incidence of from 30 to 45°. It is not problematic on the front face of the lens, since the major part of the UV radiation which comes from the front of the wearer and might attain the wearer's eye (normal incidence, 0 to 15°) generally get absorbed by the ophthalmic lens substrate. A better protection against UV radiation transmission may be obtained through solar ophthalmic lenses, which are studied and designed to reduce the visible spectrum luminosity, totally absorb UVB and totally or partially absorb UVA.
On the other hand, the UV radiation resulting from light sources located behind the wearer may reflect on the lens rear face and reach the wearer's eye if the lens is not provided with an antireflective coating which is efficient in the ultraviolet region, thus potentially affecting the wearer's health. Such phenomenon is made stronger by the trend for fashion sunglasses with high diameters which increase the risk of stray reflections getting into the eyes.
It is admitted that the light rays that may reflect onto the lens rear face and reach the wearer's eye have a narrow incidence angle range, ranging from 30 to 45° (oblique incidence).
There is currently no standard relating to the UV radiation reflection from the rear face.
There are a number of patents dealing with methods for making antireflective coatings that would be efficient in the ultraviolet region, would transmit and/or absorb the UV radiation rather than reflect it. However, optimizing the antireflective performances over the whole ultraviolet region reveals generally detrimental to the antireflective performances in the visible region. Conversely, optimizing only the antireflective performances in the visible region does not make sure that satisfactory antireflective properties can be obtained in the ultraviolet region.
The application EP 1 174 734 discloses a spectacle lens comprising on its rear face a multilayered antireflective coating designed in such a way that the reflection on the antireflective coating surface be lower as compared to the one on the bare optical article surface within the 280-700 nm wavelength range. The function of this antireflective coating consists in minimizing the reflection of the UV radiation originating from behind the wearer or reflected by the wearer's face, on the lens rear face, so as to prevent the same from reaching the wearer's eye.
The antireflective coatings described in this application are very efficient in the ultraviolet region. However, it would be advisable to improve their mean reflection factors in the visible region. Moreover, the conceived stacks are sometimes relatively sophisticated, as they may have up to 10 layers.
The more numerous the number of layers, the easier the production of an efficient antireflective coating within a broad wavelength range. However, making such complicated antireflective coatings is not so interesting from the economical point of view, since it requires a higher amount of materials and makes the industrial process last longer.
The application WO 97/28467 discloses a transparent photochromic article comprising a photochromic substrate coated with a tetra-layer-antireflective stack HI/LI/HI/LI, where HI refers to a layer with a high refractive index and LI to a layer with a low refractive index. Such coating is designed so as not to interfere with the behavior of the photochromic compounds that are present in or onto the substrate, by minimizing the reflection between 350 and 400 nm, which is the wavelength range enabling their activation. The thus prepared antireflective coatings are efficient in the UVA region, but this property is accompanied with a significant decrease in the antireflective performances in the visible region.
The patent U.S. Pat. No. 4,852,974 discloses an optical article comprising a photochromic substrate and a multilayered antireflective coating, having a mean reflection factor between 290 nm and 330 nm higher than 15% and a mean reflection factor between 330 nm and 380 nm that is lower than 4% for an angle of incidence that was not communicated. Such antireflective coating makes it possible to extend the life-time of the photochromic compounds contained in the substrate, but is relatively inefficient within a range where the UV radiation relative efficiency is the highest (290-300 nm). Moreover, it would be desirable to improve its performances in the visible region.
The application WO 2010/125 667 discloses a spectacle lens provided on its rear face with an antireflective coating enabling to reduce the reflection on the lens rear face of the UV radiation originating from behind the wearer, so that it cannot reach his eye.